1. Field of the Invention
This invention relates to an automatic transmission control system for an automobile, and more particularly, to an improvement of an automatic transmission control system in which shift control for a specific gear shift is performed by especially controlling a hydraulic working pressure.
2. Description of the Related Art
Automatic transmissions for automotive vehicles, which typically comprise a torque converter and a transmission gear mechanism, are automatically shifted into desired gears by selectively locking and unlocking a plurality of friction coupling elements such as clutches and brakes to switch the torque transmission path of the transmission gear mechanism. Such an automatic transmission is provided with hydraulic pressure control systems which supplies a pressurized working fluid to and discharges a pressurized working fluid from the respective friction coupling elements to selectively lock and unlock them. This type of automatic transmission includes, in addition to regular types of friction coupling elements, a servo cylinder type of friction coupling element provided with a band brake. This servo cylinder type friction coupling element has a servo cylinder which is divided into two pressure chambers, namely a servo apply pressure chamber and a servo release pressure chamber, by means of a spring loaded piston ordinarily forced in such a direction as to move toward the servo release pressure chamber. The servo cylinder type friction coupling element is locked when only the servo apply pressure chamber is supplied with a pressurized working fluid, and unlocked when both servo apply pressure chamber and servo release pressure chamber are not supplied with a pressurized working fluid, when both servo apply pressure chamber and servo release pressure chamber are supplied with a pressurized working fluid, and when only the servo release pressure chamber is supplied with a pressurized working fluid.
Some of available gear shifts have a necessity of locking a specific friction coupling element simultaneously with unlocking another specific friction coupling element. For example, in the case where the servo cylinder type friction coupling element is employed as a 2-4 brake to be locked in a second gear and in a fourth gear, and a single pressure chamber friction coupling element is employed as a 3-4 clutch to be locked in a third gear and in the fourth gear, it is necessary for a 2-3 gear shift to lock the 3-4 clutch simultaneously with unlocking the 2-4 brake. In this instance, because the pressurized working fluid is simultaneously supplied to both 2-4 brake and 3-4 clutch, the hydraulic pressure control circuit has fluid paths which branch off from a common fluid path and lead to the pressure chambers of these 2-4 brake and 3-4 clutch, respectively and is provided with a single hydraulic pressure regulating valve, such as a duty solenoid valve, located at the junction between the branch fluid paths and common fluid path in order to control supply of the pressurized working fluid to the pressure chambers of the 2-4 brake and 3-4 clutch. Such a hydraulic pressure control system for an automotive automatic transmission is known from, for example, Japanese Patent Publication No. 6-21643.
In the prior art hydraulic pressure control system, while the 3-4 clutch is supplied with a pressurized working fluid (which is referred to as a clutch locking fluid pressure), the 2-4 brake at the servo release pressure chamber is simultaneously supplied with a pressurized working fluid (which is referred to as a servo releasing fluid pressure) to force the piston in such a direction as to release the 2-4 brake. It is hard for the hydraulic pressure control system to perform precise control of the pressurized working fluid during a movement of the piston, and hence it is impossible to perform precise gear shift control which causes the turbine speed to fall keeping the change rate of turbine speed in agreement with a target rate through the control of 3-4 clutch locking pressure during a gear shift.
In regard to this, it may be done to control the 3-4 clutch locking fluid pressure indirectly through the servo releasing fluid pressure by controlling a pressurized working fluid in the servo apply pressure chamber of the 2-4 brake (which is referred to as a servo applying fluid pressure). In this case, even while the piston moves in the servo cylinder of the 2-4 brake, it is enabled to control the 3-4 clutch locking fluid pressure so as to cause, for example, an appropriate change in turbine speed during a gear shift. When controlling the 3-4 clutch locking fluid pressure indirectly by means of the servo applying fluid pressure, the servo applying fluid pressure, which is at a relatively high pressure level and has been supplied to provide a second gear, is fallen once with an effect of causing smooth control of the 3-4 clutch locking fluid pressure, and kept at the fallen pressure level to provide a torque phase. Afterward, when the gear shift is actually activated, in other words when the gear shift control enters on an inertia phase during which the turbine speed falls, the control of servo applying fluid pressure is performed so as to cause a satisfactory 3-4 clutch locking action.
During this control of servo applying fluid pressure, a serious problem is encountered. Specifically, when the servo applying fluid pressure is fallen for the purpose of controlling the 3-4 clutch locking fluid pressure, it is set to a pressure level suitable for the 3-4 clutch to be smoothly locked which is a specific pressure level meeting to, for example, a turbine speed change rate yielded due to input torque to the 3-4 clutch and a locking action of the 3-4 clutch. In some cases, the specific pressure level is insufficient against input torque to the 2-4 brake, allowing slippage of the 2-4 brake in advance of initiation of the 3-4 clutch locking action. This advanced slippage of the 2-4 brake, which is significant especially in the case where, while the specific pressure level of 3-4 clutch locking fluid pressure is made low due to a low turbine speed change rate when the engine is operating in a range of lower speeds, the 2-4 brake input torque increases more than the specific pressure level of 3-4 clutch locking fluid pressure increases in accordance with engine loads as a result of having entered an operating range of higher engine loads, yields an engine blow.